1. Field of the invention
The present invention relates to copolymers containing N-vinyllactam derivatives for use in microlithography, methods for preparing the same and photoresists prepared from the same. More particularly, the present invention is concerned with copolymers containing N-vinyllactam derivatives protected at 3-position, methods for preparing the copolymers, and photoresists suitable for deep uv exposure, which takes advantage of the radiation sensitivity of the copolymer to form a relief image of high sensitivity and resolution by use of deep uv.
2. Description of the Prior Art
Generally, a photoresist comprises an alkali-soluble phenol- (or cresol-) formaldehyde novolak resin and a substituted naphthoquinone diazine compound as a photosensitive material (photoactive ingredient), as reported in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470.
While the novolak resin used in such photoresist is dissolved in an aqueous alkali solution, the naphthoquinone photosensitive material acts as a dissolution inhibitor of the resist. However, when a substrate coated with the photoresist is selectively subjected to chemical radiation, the photosensitive agent is induced to be suffered from such a structural modification that the photoresist coating is more solubilized by alkali at exposed region than at unexposed region. By virtue of such difference in solubility, a relief pattern can be formed on the coating of the substrate. That is, when the substrate is immersed in an alkaline developing solution, while the unexposed region is little affected, the exposed region of the photoresist coating is dissolved, forming a pattern. However, the above-mentioned novolak type resists were found to be unsuitable to the stepper utilizing shorter wavelength, which will be described later, because they show high optical absorbance in a range of deep ultraviolet light, 200 to 300 nm.
In order to accomplish high sensitivity in the lithography process of semiconductor manufacture, chemical amplification resist has recently been developed. Indeed, the chemical amplification resist has been in the limelight since it was found to have the capacity to increase sensitivity 100-fold more comparing with conventional positive novolak resists. A chemical amplification resist, which takes advantage of the photoacid generator, is generally prepared by formulating the photoacid generator in a matrix polymer of a structure sensitively reacting to the acid. For the mechanism of the photoreaction, when the photoacid generator is exposed to light or irradiated by a high energy beam, such as X-ray beams, and electron beams, protons and strong Bronsted acid, are generated, causing the main chain or the side chain of the matrix polymer to react toward decomposition, crosslinking or a large change in polarity. This action of the acid induces, at the irradiated region, the solubility therein in the given developing solution to be altered, that is, increased or decreased. As a result, fine patterns can be formed.
As the photoacid generators, onium salts which are able to respond to light and radiation are known. Typical onium salts are ammonium salts, oxonium salts and sulfonium salts. Recently, it has been reported that organic sulfonic ester can function as a photoacid generator.
Available for the matrix polymer which can react with acid are those which are substituted a side chain which can be decomposed into carboxylic acid, phenol or alcoholic functional group by acid. T-Butylester, t-butylcarbonate, t-butoxy and t-butoxycarbonyl groups are known as the suitable substituents. Among these groups, t-butoxycarbonyl group is found to be best in sensitivity.
Such an acid-reactive polymer in a protected state or prior to reaction with acid, can be dissolved in organic solvent but not in alkali aqueous solution. However, if the acid-reactive polymer is deprotected by reaction with acid, it is soluble in alkali aqueous solution because its polarity is largely changed.
Taking advantage of this principle, the development of chemical amplification resists has been a controversial hot issue in recent years. T-Butoxycarbonyl-protected polyvinylphenol is reported to be one of the possible resins, as introduced in U.S. Pat. Nos. 4,491,628, 4,405,708 and 4,311,782.
A recent trend in submicrolithography is to use deep uv (wavelength 200 to 300 nm) as a light source, preferably, a KrF excimer laser of high power (wavelength 248), rather than conventional uv, e.g. g-line (wavelength 436 nm) or i-line (wavelength 365 nm), in order to accomplish high sensitivity and resolution. However, the chemical amplification resists are readily contaminated by the base materials present in air, raising a problem of stability in the post-exposure delay (hereinafter referred to as "PED") at which T-top is formed on the course of fine pattern formation.
Various methods have been suggested to improve the PED stability. Among these methods, use of base additives (mainly amines) was found to bring an improvement in PED stability, but was disadvantageous in that it caused a decrease in radiation-sensitivity of photoresist and the base additives were not uniformly distributed in film because of their diffusion into the film surface during processing.